1250 lines
30 KiB
ReStructuredText
1250 lines
30 KiB
ReStructuredText
=================
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Hy (the language)
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=================
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.. warning::
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This is incomplete; please consider contributing to the documentation
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effort.
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Theory of Hy
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============
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Hy maintains, over everything else, 100% compatibility in both directions
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with Python itself. All Hy code follows a few simple rules. Memorize
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this, it's going to come in handy.
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These rules help make sure code is idiomatic and interface-able in both
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languages.
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* Symbols in earmufs will be translated to the uppercased version of that
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string. For example, `*foo*` will become `FOO`.
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* UTF-8 entities will be encoded using
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`punycode <http://en.wikipedia.org/wiki/Punycode>`_ and prefixed with
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`hy_`. For instance, `⚘` will become `hy_w7h`, `♥` will become `hy_g6h`,
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and `i♥u` will become `hy_iu_t0x`.
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* Symbols that contain dashes will have them replaced with underscores. For
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example, `render-template` will become `render_template`. This means that
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symbols with dashes will shadow their underscore equivalents, and vice
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versa.
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Builtins
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========
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Hy features a number of special forms that are used to help generate
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correct Python AST. The following are "special" forms, which may have
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behavior that's slightly unexpected in some situations.
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.
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-
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.. versionadded:: 0.9.13
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`.` is used to perform attribute access on objects. It uses a small DSL
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to allow quick access to attributes and items in a nested datastructure.
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For instance,
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.. code-block:: clj
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(. foo bar baz [(+ 1 2)] frob)
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Compiles down to
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.. code-block:: python
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foo.bar.baz[1 + 2].frob
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`.` compiles its first argument (in the example, `foo`) as the object on
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which to do the attribute dereference. It uses bare symbols as
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attributes to access (in the example, `bar`, `baz`, `frob`), and
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compiles the contents of lists (in the example, ``[(+ 1 2)]``) for
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indexation. Other arguments throw a compilation error.
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Access to unknown attributes throws an :exc:`AttributeError`. Access to
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unknown keys throws an :exc:`IndexError` (on lists and tuples) or a
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:exc:`KeyError` (on dicts).
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->
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--
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`->` or `threading macro` is used to avoid nesting of expressions. The threading
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macro inserts each expression into the next expression’s first argument place.
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The following code demonstrates this:
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.. code-block:: clj
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=> (defn output [a b] (print a b))
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=> (-> (+ 5 5) (output 5))
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10 5
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->>
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---
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`->>` or `threading tail macro` is similar to `threading macro` but instead of
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inserting each expression into the next expression’s first argument place, it
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appends it as the last argument. The following code demonstrates this:
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.. code-block:: clj
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=> (defn output [a b] (print a b))
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=> (->> (+ 5 5) (output 5))
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5 10
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apply
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-----
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`apply` is used to apply an optional list of arguments and an optional
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dictionary of kwargs to a function.
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Usage: `(apply fn-name [args] [kwargs])`
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Examples:
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.. code-block:: clj
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(defn thunk []
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"hy there")
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(apply thunk)
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;=> "hy there"
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(defn total-purchase [price amount &optional [fees 1.05] [vat 1.1]]
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(* price amount fees vat))
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(apply total-purchase [10 15])
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;=> 173.25
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(apply total-purchase [10 15] {"vat" 1.05})
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;=> 165.375
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(apply total-purchase [] {"price" 10 "amount" 15 "vat" 1.05})
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;=> 165.375
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and
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---
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`and` form is used in logical expressions. It takes at least two parameters. If
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all parameters evaluate to `True` the last parameter is returned. In any other
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case the first false value will be returned. Examples of usage:
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.. code-block:: clj
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=> (and True False)
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False
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=> (and True True)
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True
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=> (and True 1)
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1
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=> (and True [] False True)
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[]
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.. note:: `and` shortcuts and stops evaluating parameters as soon as the first
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false is encountered. However, in the current implementation of Hy
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statements are executed as soon as they are converted to expressions.
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The following two examples demonstrates the difference.
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.. code-block:: clj
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=> (and False (print "hello"))
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hello
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False
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=> (defn side-effects [x] (print "I can has" x) x)
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=> (and (side-effects false) (side-effects 42))
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I can has False
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False
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assert
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------
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`assert` is used to verify conditions while the program is running. If the
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condition is not met, an `AssertionError` is raised. The example usage:
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.. code-block:: clj
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(assert (= variable expected-value))
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Assert takes a single parameter, a conditional that evaluates to either `True`
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or `False`.
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assoc
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-----
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`assoc` form is used to associate a key with a value in a dictionary or to set
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an index of a list to a value. It takes at least three parameters: `datastructure`
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to be modified, `key` or `index` and `value`. If more than three parameters are
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used it will associate in pairs.
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Examples of usage:
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.. code-block:: clj
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=>(let [[collection {}]]
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... (assoc collection "Dog" "Bark")
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... (print collection))
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{u'Dog': u'Bark'}
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=>(let [[collection {}]]
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... (assoc collection "Dog" "Bark" "Cat" "Meow")
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... (print collection))
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{u'Cat': u'Meow', u'Dog': u'Bark'}
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=>(let [[collection [1 2 3 4]]]
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... (assoc collection 2 None)
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... (print collection))
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[1, 2, None, 4]
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.. note:: `assoc` modifies the datastructure in place and returns `None`.
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break
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-----
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`break` is used to break out from a loop. It terminates the loop immediately.
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The following example has an infinite while loop that is terminated as soon as
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the user enters `k`.
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.. code-block:: clj
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(while True (if (= "k" (raw-input "? "))
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(break)
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(print "Try again")))
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cond
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----
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`cond` macro can be used to build nested if-statements.
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The following example shows the relationship between the macro and the expanded
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code:
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.. code-block:: clj
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(cond [condition-1 result-1]
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[condition-2 result-2])
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(if condition-1 result-1
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(if condition-2 result-2))
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As shown below only the first matching result block is executed.
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.. code-block:: clj
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=> (defn check-value [value]
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... (cond [(< value 5) (print "value is smaller than 5")]
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... [(= value 5) (print "value is equal to 5")]
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... [(> value 5) (print "value is greater than 5")]
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... [True (print "value is something that it should not be")]))
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=> (check-value 6)
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value is greater than 5
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continue
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--------
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`continue` returns execution to the start of a loop. In the following example,
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function `(side-effect1)` is called for each iteration. `(side-effect2)`
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however is called only for every other value in the list.
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.. code-block:: clj
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;; assuming that (side-effect1) and (side-effect2) are functions and
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;; collection is a list of numerical values
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(for [x collection]
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(do
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(side-effect1 x)
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(if (% x 2)
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(continue))
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(side-effect2 x)))
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do / progn
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----------
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the `do` and `progn` forms are used to evaluate each of their arguments and
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return the last one. Return values from every other than the last argument are
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discarded. It can be used in `lambda` or `list-comp` to perform more complex
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logic as shown by one of the examples.
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Some example usage:
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.. code-block:: clj
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=> (if true
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... (do (print "Side effects rock!")
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... (print "Yeah, really!")))
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Side effects rock!
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Yeah, really!
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;; assuming that (side-effect) is a function that we want to call for each
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;; and every value in the list, but which return values we do not care
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=> (list-comp (do (side-effect x)
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... (if (< x 5) (* 2 x)
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... (* 4 x)))
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... (x (range 10)))
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[0, 2, 4, 6, 8, 20, 24, 28, 32, 36]
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`do` can accept any number of arguments, from 1 to n.
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def / setv
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-----------------
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`def` and `setv` are used to bind value, object or a function to a symbol. For
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example:
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.. code-block:: clj
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=> (def names ["Alice" "Bob" "Charlie"])
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=> (print names)
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[u'Alice', u'Bob', u'Charlie']
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=> (setv counter (fn [collection item] (.count collection item)))
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=> (counter [1 2 3 4 5 2 3] 2)
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2
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defclass
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--------
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new classes are declared with `defclass`. It can takes two optional parameters:
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a vector defining a possible super classes and another vector containing
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attributes of the new class as two item vectors.
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.. code-block:: clj
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(defclass class-name [super-class-1 super-class-2]
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[[attribute value]])
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Both values and functions can be bound on the new class as shown by the example
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below:
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.. code-block:: clj
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=> (defclass Cat []
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... [[age None]
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... [colour "white"]
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... [speak (fn [self] (print "Meow"))]])
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=> (def spot (Cat))
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=> (setv spot.colour "Black")
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'Black'
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=> (.speak spot)
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Meow
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.. _defn:
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defn / defun
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------------
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`defn` and `defun` macros are used to define functions. They take three
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parameters: `name` of the function to define, vector of `parameters` and the
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`body` of the function:
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|
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.. code-block:: clj
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(defn name [params] body)
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Parameters may have following keywords in front of them:
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&optional
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parameter is optional. The parameter can be given as a two item list, where
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the first element is parameter name and the second is the default value. The
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parameter can be also given as a single item, in which case the default
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value is None.
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.. code-block:: clj
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=> (defn total-value [value &optional [value-added-tax 10]]
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... (+ (/ (* value value-added-tax) 100) value))
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=> (total-value 100)
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110.0
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=> (total-value 100 1)
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101.0
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&key
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&kwargs
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parameter will contain 0 or more keyword arguments.
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|
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The following code examples defines a function that will print all keyword
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arguments and their values.
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.. code-block:: clj
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=> (defn print-parameters [&kwargs kwargs]
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... (for [(, k v) (.items kwargs)] (print k v)))
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=> (kwapply (print-parameters) {"parameter-1" 1 "parameter-2" 2})
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parameter-2 2
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parameter-1 1
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&rest
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parameter will contain 0 or more positional arguments. No other positional
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arguments may be specified after this one.
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|
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The following code example defines a function that can be given 0 to n
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numerical parameters. It then sums every odd number and substracts
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every even number.
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.. code-block:: clj
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=> (defn zig-zag-sum [&rest numbers]
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(let [[odd-numbers (list-comp x [x numbers] (odd? x))]
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[even-numbers (list-comp x [x numbers] (even? x))]]
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(- (sum odd-numbers) (sum even-numbers))))
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=> (zig-zag-sum)
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0
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=> (zig-zag-sum 3 9 4)
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8
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=> (zig-zag-sum 1 2 3 4 5 6)
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-3
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.. _defn-alias / defun-alias:
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defn-alias / defun-alias
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------------------------
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.. versionadded:: 0.9.13
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The `defn-alias` and `defun-alias` macros are much like `defn`_ above,
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with the difference that instead of defining a function with a single
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name, these can also define aliases. Other than taking a list of
|
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symbols for function names as the first parameter, `defn-alias` and
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||
`defun-alias` have no other differences compared to `defn` and
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`defun`.
|
||
|
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.. code-block:: clj
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=> (defn-alias [main-name alias] []
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... (print "Hello!"))
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=> (main-name)
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"Hello!"
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=> (alias)
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"Hello!"
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.. _defmacro:
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defmacro
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--------
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`defmacro` is used to define macros. The general format is
|
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`(defmacro name [parameters] expr)`.
|
||
|
||
The following example defines a macro that can be used to swap order of elements in
|
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code, allowing the user to write code in infix notation, where operator is in
|
||
between the operands.
|
||
|
||
.. code-block:: clj
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||
|
||
=> (defmacro infix [code]
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... (quasiquote (
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... (unquote (get code 1))
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... (unquote (get code 0))
|
||
... (unquote (get code 2)))))
|
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|
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=> (infix (1 + 1))
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2
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||
|
||
.. _defmacro-alias:
|
||
|
||
defmacro-alias
|
||
--------------
|
||
|
||
`defmacro-alias` is used to define macros with multiple names
|
||
(aliases). The general format is `(defmacro-alias [names] [parameters]
|
||
expr)`. It creates multiple macros with the same parameter list and
|
||
body, under the specified list of names.
|
||
|
||
The following example defines two macros, both of which allow the user
|
||
to write code in infix notation.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defmacro-alias [infix infi] [code]
|
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... (quasiquote (
|
||
... (unquote (get code 1))
|
||
... (unquote (get code 0))
|
||
... (unquote (get code 2)))))
|
||
|
||
=> (infix (1 + 1))
|
||
2
|
||
=> (infi (1 + 1))
|
||
2
|
||
|
||
.. _defmacro/g!:
|
||
|
||
defmacro/g!
|
||
------------
|
||
|
||
.. versionadded:: 0.9.12
|
||
|
||
`defmacro/g!` is a special version of `defmacro` that is used to
|
||
automatically generate :ref:`gensym` for any symbol that
|
||
starts with ``g!``.
|
||
|
||
So ``g!a`` would become ``(gensym "a")``.
|
||
|
||
.. seealso::
|
||
|
||
Section :ref:`using-gensym`
|
||
|
||
defreader
|
||
---------
|
||
|
||
.. versionadded:: 0.9.12
|
||
|
||
`defreader` defines a reader macro, enabling you to restructure or
|
||
modify syntax.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defreader ^ [expr] (print expr))
|
||
=> #^(1 2 3 4)
|
||
(1 2 3 4)
|
||
=> #^"Hello"
|
||
"Hello"
|
||
|
||
.. seealso::
|
||
|
||
Section :ref:`Reader Macros <reader-macros>`
|
||
|
||
del
|
||
---
|
||
|
||
.. versionadded:: 0.9.12
|
||
|
||
`del` removes an object from the current namespace.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (setv foo 42)
|
||
=> (del foo)
|
||
=> foo
|
||
Traceback (most recent call last):
|
||
File "<console>", line 1, in <module>
|
||
NameError: name 'foo' is not defined
|
||
|
||
`del` can also remove objects from a mapping, a list, ...
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (setv test (list (range 10)))
|
||
=> test
|
||
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
=> (del (slice test 2 4)) ;; remove items from 2 to 4 excluded
|
||
=> test
|
||
[0, 1, 4, 5, 6, 7, 8, 9]
|
||
=> (setv dic {"foo" "bar"})
|
||
=> dic
|
||
{"foo": "bar"}
|
||
=> (del (get dic "foo"))
|
||
=> dic
|
||
{}
|
||
|
||
eval
|
||
----
|
||
|
||
`eval` evaluates a quoted expression and returns the value.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (eval '(print "Hello World"))
|
||
"Hello World"
|
||
|
||
eval-and-compile
|
||
----------------
|
||
|
||
|
||
eval-when-compile
|
||
-----------------
|
||
|
||
|
||
first / car
|
||
-----------
|
||
|
||
`first` and `car` are macros for accessing the first element of a collection:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (first (range 10))
|
||
0
|
||
|
||
|
||
for
|
||
-------
|
||
|
||
`for` is used to call a function for each element in a list or vector.
|
||
The results of each call are discarded and the for expression returns
|
||
None instead. The example code iterates over `collection` and
|
||
for each `element` in `collection` calls the `side-effect`
|
||
function with `element` as its argument:
|
||
|
||
.. code-block:: clj
|
||
|
||
;; assuming that (side-effect) is a function that takes a single parameter
|
||
(for [element collection] (side-effect element))
|
||
|
||
;; for can have an optional else block
|
||
(for [element collection] (side-effect element)
|
||
(else (side-effect-2)))
|
||
|
||
The optional `else` block is executed only if the `for` loop terminates
|
||
normally. If the execution is halted with `break`, the `else` does not execute.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (for [element [1 2 3]] (if (< element 3)
|
||
... (print element)
|
||
... (break))
|
||
... (else (print "loop finished")))
|
||
1
|
||
2
|
||
|
||
=> (for [element [1 2 3]] (if (< element 4)
|
||
... (print element)
|
||
... (break))
|
||
... (else (print "loop finished")))
|
||
1
|
||
2
|
||
3
|
||
loop finished
|
||
|
||
|
||
.. _gensym:
|
||
|
||
gensym
|
||
------
|
||
|
||
.. versionadded:: 0.9.12
|
||
|
||
`gensym` form is used to generate a unique symbol to allow writing macros
|
||
without accidental variable name clashes.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (gensym)
|
||
u':G_1235'
|
||
|
||
=> (gensym "x")
|
||
u':x_1236'
|
||
|
||
.. seealso::
|
||
|
||
Section :ref:`using-gensym`
|
||
|
||
get
|
||
---
|
||
|
||
`get` form is used to access single elements in lists and dictionaries. `get`
|
||
takes two parameters, the `datastructure` and the `index` or `key` of the item.
|
||
It will then return the corresponding value from the dictionary or the list.
|
||
Example usages:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (let [[animals {"dog" "bark" "cat" "meow"}]
|
||
... [numbers ["zero" "one" "two" "three"]]]
|
||
... (print (get animals "dog"))
|
||
... (print (get numbers 2)))
|
||
bark
|
||
two
|
||
|
||
.. note:: `get` raises a KeyError if a dictionary is queried for a non-existing
|
||
key.
|
||
|
||
.. note:: `get` raises an IndexError if a list or a tuple is queried for an index
|
||
that is out of bounds.
|
||
|
||
|
||
global
|
||
------
|
||
|
||
`global` can be used to mark a symbol as global. This allows the programmer to
|
||
assign a value to a global symbol. Reading a global symbol does not require the
|
||
`global` keyword, just the assigning does.
|
||
|
||
Following example shows how global `a` is assigned a value in a function and later
|
||
on printed on another function. Without the `global` keyword, the second function
|
||
would thrown a `NameError`.
|
||
|
||
.. code-block:: clj
|
||
|
||
(defn set-a [value]
|
||
(global a)
|
||
(setv a value))
|
||
|
||
(defn print-a []
|
||
(print a))
|
||
|
||
(set-a 5)
|
||
(print-a)
|
||
|
||
if / if-not
|
||
-----------
|
||
|
||
the `if` form is used to conditionally select code to be executed. It has to
|
||
contain the condition block and the block to be executed if the condition
|
||
evaluates `True`. Optionally it may contain a block that is executed in case
|
||
the evaluation of the condition is `False`. The `if-not` form (*new in
|
||
0.9.13*) is similar, but the first block after the test will be
|
||
executed when the test fails, while the other, conditional one, when
|
||
the test succeeds - opposite of the order of the `if` form.
|
||
|
||
Example usage:
|
||
|
||
.. code-block:: clj
|
||
|
||
(if (money-left? account)
|
||
(print "lets go shopping")
|
||
(print "lets go and work"))
|
||
|
||
(if-not (money-left? account)
|
||
(print "lets go and work")
|
||
(print "lets go shopping"))
|
||
|
||
Truth values of Python objects are respected. Values `None`, `False`, zero of
|
||
any numeric type, empty sequence and empty dictionary are considered `False`.
|
||
Everything else is considered `True`.
|
||
|
||
|
||
import
|
||
------
|
||
|
||
`import` is used to import modules, like in Python. There are several forms
|
||
of import you can use.
|
||
|
||
.. code-block:: clj
|
||
|
||
;; Imports each of these modules
|
||
;;
|
||
;; Python:
|
||
;; import sys
|
||
;; import os.path
|
||
(import sys os.path)
|
||
|
||
;; Import from a module
|
||
;;
|
||
;; Python: from os.path import exists, isdir, isfile
|
||
(import [os.path [exists isdir isfile]])
|
||
|
||
;; Import with an alias
|
||
;;
|
||
;; Python: import sys as systest
|
||
(import [sys :as systest])
|
||
|
||
;; You can list as many imports as you like of different types.
|
||
(import [tests.resources [kwtest function-with-a-dash]]
|
||
[os.path [exists isdir isfile]]
|
||
[sys :as systest])
|
||
|
||
|
||
kwapply
|
||
-------
|
||
|
||
`kwapply` can be used to supply keyword arguments to a function.
|
||
|
||
For example:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defn rent-car [&kwargs kwargs]
|
||
... (cond [(in :brand kwargs) (print "brand:" (:brand kwargs))]
|
||
... [(in :model kwargs) (print "model:" (:model kwargs))]))
|
||
|
||
=> (kwapply (rent-car) {:model "T-Model"})
|
||
model: T-Model
|
||
|
||
=> (defn total-purchase [price amount &optional [fees 1.05] [vat 1.1]]
|
||
... (* price amount fees vat))
|
||
|
||
=> (total-purchase 10 15)
|
||
173.25
|
||
|
||
=> (kwapply (total-purchase 10 15) {"vat" 1.05})
|
||
165.375
|
||
|
||
|
||
lambda / fn
|
||
-----------
|
||
|
||
`lambda` and `fn` can be used to define an anonymous function. The parameters are
|
||
similar to `defn`: first parameter is vector of parameters and the rest is the
|
||
body of the function. lambda returns a new function. In the example an anonymous
|
||
function is defined and passed to another function for filtering output.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (def people [{:name "Alice" :age 20}
|
||
... {:name "Bob" :age 25}
|
||
... {:name "Charlie" :age 50}
|
||
... {:name "Dave" :age 5}])
|
||
|
||
=> (defn display-people [people filter]
|
||
... (for [person people] (if (filter person) (print (:name person)))))
|
||
|
||
=> (display-people people (fn [person] (< (:age person) 25)))
|
||
Alice
|
||
Dave
|
||
|
||
|
||
let
|
||
---
|
||
|
||
`let` is used to create lexically scoped variables. They are created at the
|
||
beginning of `let` form and cease to exist after the form. The following
|
||
example showcases this behaviour:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (let [[x 5]] (print x)
|
||
... (let [[x 6]] (print x))
|
||
... (print x))
|
||
5
|
||
6
|
||
5
|
||
|
||
`let` macro takes two parameters: a vector defining `variables` and `body`,
|
||
which is being executed. `variables` is a vector where each element is either
|
||
a single variable or a vector defining a variable value pair. In case of a
|
||
single variable, it is assigned value None, otherwise the supplied value is
|
||
used.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (let [x [y 5]] (print x y))
|
||
None 5
|
||
|
||
|
||
list-comp
|
||
---------
|
||
|
||
`list-comp` performs list comprehensions. It takes two or three parameters.
|
||
The first parameter is the expression controlling the return value, while
|
||
the second is used to select items from a list. The third and optional
|
||
parameter can be used to filter out some of the items in the list based on a
|
||
conditional expression. Some examples:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (def collection (range 10))
|
||
=> (list-comp x [x collection])
|
||
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
=> (list-comp (* x 2) [x collection])
|
||
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
|
||
|
||
=> (list-comp (* x 2) [x collection] (< x 5))
|
||
[0, 2, 4, 6, 8]
|
||
|
||
|
||
not
|
||
---
|
||
|
||
`not` form is used in logical expressions. It takes a single parameter and
|
||
returns a reversed truth value. If `True` is given as a parameter, `False`
|
||
will be returned and vice-versa. Examples for usage:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (not True)
|
||
False
|
||
|
||
=> (not False)
|
||
True
|
||
|
||
=> (not None)
|
||
True
|
||
|
||
|
||
or
|
||
--
|
||
|
||
`or` form is used in logical expressions. It takes at least two parameters. It
|
||
will return the first non-false parameter. If no such value exist, the last
|
||
parameter will be returned.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (or True False)
|
||
True
|
||
|
||
=> (and False False)
|
||
False
|
||
|
||
=> (and False 1 True False)
|
||
1
|
||
|
||
.. note:: `or` shortcuts and stops evaluating parameters as soon as the first
|
||
true is encountered. However, in the current implementation of Hy
|
||
statements are executed as soon as they are converted to expressions.
|
||
The following two examples demonstrates the difference.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (or True (print "hello"))
|
||
hello
|
||
True
|
||
|
||
=> (defn side-effects [x] (print "I can has" x) x)
|
||
=> (or (side-effects 42) (side-effects False))
|
||
I can has 42
|
||
42
|
||
|
||
|
||
print
|
||
-----
|
||
|
||
the `print` form is used to output on screen. Example usage:
|
||
|
||
.. code-block:: clj
|
||
|
||
(print "Hello world!")
|
||
|
||
.. note:: `print` always returns None
|
||
|
||
|
||
quasiquote
|
||
----------
|
||
|
||
`quasiquote` allows you to quote a form, but also to
|
||
selectively evaluate expressions, expressions inside a `quasiquote`
|
||
can be selectively evaluated using `unquote` (~). The evaluated form can
|
||
also be spliced using `unquote-splice` (~@). Quasiquote can be also written
|
||
using the backquote (`) symbol.
|
||
|
||
|
||
.. code-block:: clj
|
||
|
||
;; let `qux' be a variable with value (bar baz)
|
||
`(foo ~qux)
|
||
; equivalent to '(foo (bar baz))
|
||
`(foo ~@qux)
|
||
; equivalent to '(foo bar baz)
|
||
|
||
|
||
quote
|
||
-----
|
||
|
||
`quote` returns the form passed to it without evaluating. `quote` can
|
||
be alternatively written using the (') symbol
|
||
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (setv x '(print "Hello World"))
|
||
; variable x is set to expression & not evaluated
|
||
=> x
|
||
(u'print' u'Hello World')
|
||
=> (eval x)
|
||
Hello World
|
||
|
||
|
||
require
|
||
-------
|
||
|
||
`require` is used to import macros from a given module. It takes at least one
|
||
parameter specifying the module which macros should be imported. Multiple
|
||
modules can be imported with a single `require`.
|
||
|
||
The following example will import macros from `module-1` and `module-2`:
|
||
|
||
.. code-block:: clj
|
||
|
||
(require module-1 module-2)
|
||
|
||
|
||
rest / cdr
|
||
----------
|
||
|
||
`rest` and `cdr` return the collection passed as an argument without the first
|
||
element:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (rest (range 10))
|
||
[1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
|
||
slice
|
||
-----
|
||
|
||
`slice` can be used to take a subset of a list and create a new list from it.
|
||
The form takes at least one parameter specifying the list to slice. Two
|
||
optional parameters can be used to give the start and end position of the
|
||
subset. If they are not supplied, default value of None will be used instead.
|
||
Third optional parameter is used to control step between the elements.
|
||
|
||
`slice` follows the same rules as the Python counterpart. Negative indecies are
|
||
counted starting from the end of the list.
|
||
Some examples of
|
||
usage:
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (def collection (range 10))
|
||
|
||
=> (slice collection)
|
||
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
|
||
|
||
=> (slice collection 5)
|
||
[5, 6, 7, 8, 9]
|
||
|
||
=> (slice collection 2 8)
|
||
[2, 3, 4, 5, 6, 7]
|
||
|
||
=> (slice collection 2 8 2)
|
||
[2, 4, 6]
|
||
|
||
=> (slice collection -4 -2)
|
||
[6, 7]
|
||
|
||
|
||
throw / raise
|
||
-------------
|
||
|
||
the `throw` or `raise` forms can be used to raise an Exception at runtime.
|
||
|
||
|
||
Example usage
|
||
|
||
.. code-block:: clj
|
||
|
||
(throw)
|
||
; re-rase the last exception
|
||
|
||
(throw IOError)
|
||
; Throw an IOError
|
||
|
||
(throw (IOError "foobar"))
|
||
; Throw an IOError("foobar")
|
||
|
||
|
||
`throw` can acccept a single argument (an `Exception` class or instance), or
|
||
no arguments to re-raise the last Exception.
|
||
|
||
|
||
try
|
||
---
|
||
|
||
the `try` form is used to start a `try` / `catch` block. The form is used
|
||
as follows
|
||
|
||
.. code-block:: clj
|
||
|
||
(try
|
||
(error-prone-function)
|
||
(catch [e ZeroDivisionError] (print "Division by zero"))
|
||
(else (print "no errors"))
|
||
(finally (print "all done")))
|
||
|
||
`try` must contain at least one `catch` block, and may optionally have an
|
||
`else` or `finally` block. If an error is raised with a matching catch
|
||
block during execution of `error-prone-function` then that catch block will
|
||
be executed. If no errors are raised the `else` block is executed. Regardless
|
||
if an error was raised or not, the `finally` block is executed as last.
|
||
|
||
|
||
unless
|
||
------
|
||
|
||
`unless` macro is a shorthand for writing a if-statement that checks if the
|
||
given conditional is False. The following shows how the macro expands into code.
|
||
|
||
.. code-block:: clj
|
||
|
||
(unless conditional statement)
|
||
|
||
(if conditional
|
||
None
|
||
(do statement))
|
||
|
||
|
||
unquote
|
||
-------
|
||
|
||
Within a quasiquoted form, `unquote` forces evaluation of a symbol. `unquote`
|
||
is aliased to the `~` symbol.
|
||
|
||
.. code-block:: clj
|
||
|
||
(def name "Cuddles")
|
||
(quasiquote (= name (unquote name)))
|
||
;=> (u'=' u'name' u'Cuddles')
|
||
|
||
`(= name ~name)
|
||
;=> (u'=' u'name' u'Cuddles')
|
||
|
||
|
||
unquote-splice
|
||
--------------
|
||
|
||
`unquote-splice` forces the evaluation of a symbol within a quasiquoted form,
|
||
much like `unquote`. `unquote-splice` can only be used when the symbol being
|
||
unquoted contains an iterable value, as it "splices" that iterable into the
|
||
quasiquoted form. `unquote-splice` is aliased to the `~@` symbol.
|
||
|
||
.. code-block:: clj
|
||
|
||
(def nums [1 2 3 4])
|
||
(quasiquote (+ (unquote-splice nums)))
|
||
;=> (u'+' 1L 2L 3L 4L)
|
||
|
||
`(+ ~@nums)
|
||
;=> (u'+' 1L 2L 3L 4L)
|
||
|
||
|
||
|
||
when
|
||
----
|
||
|
||
`when` is similar to `unless`, except it tests when the given conditional is
|
||
True. It is not possible to have an `else` block in `when` macro. The following
|
||
shows how the macro is expanded into code.
|
||
|
||
.. code-block:: clj
|
||
|
||
(when conditional statement)
|
||
|
||
(if conditional (do statement))
|
||
|
||
while
|
||
-----
|
||
|
||
`while` form is used to execute a single or more blocks as long as a condition
|
||
is being met.
|
||
|
||
The following example will output "hello world!" on screen indefinetely:
|
||
|
||
.. code-block:: clj
|
||
|
||
(while True (print "hello world!"))
|
||
|
||
|
||
with
|
||
----
|
||
|
||
`with` is used to wrap execution of a block with a context manager. The context
|
||
manager can then set up the local system and tear it down in a controlled
|
||
manner. Typical example of using `with` is processing files. `with` can bind
|
||
context to an argument or ignore it completely, as shown below:
|
||
|
||
.. code-block:: clj
|
||
|
||
(with [[arg (expr)]] block)
|
||
|
||
(with [[(expr)]] block)
|
||
|
||
(with [[arg (expr)] [(expr)]] block)
|
||
|
||
The following example will open file `NEWS` and print its content on screen. The
|
||
file is automatically closed after it has been processed.
|
||
|
||
.. code-block:: clj
|
||
|
||
(with [[f (open "NEWS")]] (print (.read f)))
|
||
|
||
|
||
with-decorator
|
||
--------------
|
||
|
||
`with-decorator` is used to wrap a function with another. The function performing
|
||
decoration should accept a single value, the function being decorated and return
|
||
a new function. `with-decorator` takes two parameters, the function performing
|
||
decoration and the function being decorated.
|
||
|
||
In the following example, `inc-decorator` is used to decorate function `addition`
|
||
with a function that takes two parameters and calls the decorated function with
|
||
values that are incremented by 1. When decorated `addition` is called with values
|
||
1 and 1, the end result will be 4 (1+1 + 1+1).
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defn inc-decorator [func]
|
||
... (fn [value-1 value-2] (func (+ value-1 1) (+ value-2 1))))
|
||
=> (with-decorator inc-decorator (defn addition [a b] (+ a b)))
|
||
=> (addition 1 1)
|
||
4
|
||
|
||
|
||
.. _with-gensyms:
|
||
|
||
with-gensyms
|
||
-------------
|
||
|
||
.. versionadded:: 0.9.12
|
||
|
||
`with-gensym` form is used to generate a set of :ref:`gensym` for use
|
||
in a macro.
|
||
|
||
.. code-block:: clojure
|
||
|
||
(with-gensyms [a b c]
|
||
...)
|
||
|
||
expands to:
|
||
|
||
.. code-block:: clojure
|
||
|
||
(let [[a (gensym)
|
||
[b (gensym)
|
||
[c (gensym)]]
|
||
...)
|
||
|
||
.. seealso::
|
||
|
||
Section :ref:`using-gensym`
|
||
|
||
|
||
yield
|
||
-----
|
||
|
||
`yield` is used to create a generator object, that returns 1 or more values.
|
||
The generator is iterable and therefore can be used in loops, list
|
||
comprehensions and other similar constructs.
|
||
|
||
The function random-numbers shows how generators can be used to generate
|
||
infinite series without consuming infinite amount of memory.
|
||
|
||
.. code-block:: clj
|
||
|
||
=> (defn multiply [bases coefficients]
|
||
... (for [[(, base coefficient) (zip bases coefficients)]]
|
||
... (yield (* base coefficient))))
|
||
|
||
=> (multiply (range 5) (range 5))
|
||
<generator object multiply at 0x978d8ec>
|
||
|
||
=> (list-comp value [value (multiply (range 10) (range 10))])
|
||
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
|
||
|
||
=> (import random)
|
||
=> (defn random-numbers [low high]
|
||
... (while True (yield (.randint random low high))))
|
||
=> (list-comp x [x (take 15 (random-numbers 1 50))])])
|
||
[7, 41, 6, 22, 32, 17, 5, 38, 18, 38, 17, 14, 23, 23, 19]
|